What Does the Acronym RSS Stand For?

I read that RSS means Really Simple Syndication and Rich Site Summary. But I also saw RDF Site Summary mentioned. Is this another valid acronym or is an error?

That is the least common acronymn definition, but it is used by some, mainly pertaining to version RSS 1.0 which is not compatible with the .9 or 2.0 series. In other words the specification is very different.